Method and assembly for gas detection via a convergent birefringent filter

Abstract

An assembly and method provide sensitive high speed spectroscopic gas detection via the use of a non-collimated convergent light path through a birefringent crystal in a birefringent filter design to allow improved light collection and a reduction in the number of optical elements required to produce a useful absorption signal for the detection of gases.

What is claimed is:  
     
       1. A detection assembly comprising: 
       a source of convergent electromagnetic radiation spaced apart from a tunable birefringent filter by a sample volume and  
       a detector to detect the incidence of electromagnetic radiation of a selected wavelength after passage of convergent electromagnetic radiation through the sample volume and the birefringent filter.  
     
     
       2. The detection assembly of  claim 1  wherein the source of convergent electromagnetic radiation comprises: 
       a source providing a supply of light and  
       a first optical element to direct at least a portion of the supply of light to form a converging beam of light.  
     
     
       3. The detection assembly of  claim 2  additionally comprising a filter interposed between the first optical element and the tunable birefringent filter. 
     
     
       4. The detection assembly of  claim 3  additionally comprising a second optical element interposed between the birefringent filter and the detector. 
     
     
       5. The detection assembly of  claim 1  wherein the tunable birefringent filter comprises at least one birefringent filter stage. 
     
     
       6. The detection assembly of  claim 1  additionally comprising a second optical element interposed between the birefringent filter and the detector. 
     
     
       7. The detection assembly of  claim 1  comprising means to control the temperature of the birefringent filter. 
     
     
       8. The detection assembly of  claim 1  comprising means to produce a signal representative of absorption due to presence of a selected target species in the sample volume. 
     
     
       9. The detection assembly in accordance with  claim 8  wherein the selected target species in the sample volume is methane. 
     
     
       10. The detection assembly of  claim 1  wherein the tunable birefringent filter is a transverse birefringent filter. 
     
     
       11. The detection assembly of  claim 1  wherein the birefringent filter comprises an input and an output polarizer with a birefringent electro-optical material interposed therebetween. 
     
     
       12. The detection assembly of  claim 11  wherein the birefringent filter additionally comprises means for applying a voltage to the birefringent electro-optical material to induce a modulating electric field therein. 
     
     
       13. In a method for detecting the presence of a target species in a sample volume wherein light is passed through the sample volume to a birefringent filter, the improvement comprising: 
       passing convergent light rays through the sample volume to the birefringent filter.  
     
     
       14. The method of  claim 13  wherein light passed through the birefringent filter is incident onto a detector. 
     
     
       15. The method of  claim 13  wherein the target species absorbs light in a predetermined spectral region, said method additionally comprising: 
       tuning the birefringent filter through the light absorbing predetermined spectral region of the target species.  
     
     
       16. The method of  claim 13  additionally comprising: 
       producing a signal representing the presence of the target species in the gas sample.  
     
     
       17. The method of  claim 13  wherein the birefringent filter comprises an input and an output polarizer with a birefringent electro-optical material interposed therebetween, said method comprising: 
       applying a voltage to the birefringent electro-optical material to induce a modulating electric field therein.  
     
     
       18. The method of  claim 13  wherein the target species is methane. 
     
     
       19. A method for detecting the presence of a target species in a gas sample, wherein the target species absorbs light in a predetermined spectral region, the method comprising: 
       passing convergent light through the gas sample to a birefringent filter and subsequently to a detector,  
       tuning the birefringent filter through the predetermined spectral region, and  
       producing a signal representing the presence of the target species in the gas sample.  
     
     
       20. The method of  claim 19  wherein the target species is methane.